Radio frequency liquid dielectric load with inner conductor and tapered shell



May M, w65 A. A. GOLDFINGER 3,183,45

RADIO FREQUENCY LIQUID DIELECTRIC LOAD WITH INNER CONDUCTOR AND TAPEREDSHELL Filed Dec. s, 1960 United States Patent() 3,183,458 RADEQFREQUENCY LIQUED DIELECTREC LAD WH'H INNER CQNDUCTQR AND TAERED SimiliArthur A. Goldfinger, Palo Alto, Calif., assigner to Eitel- McCullough,Inc., San Carlos, Calif., a corporation of Caiifornia Filed Dec. `8,1960, Ser. No. 74,571 3 Claims. (Cl. S33-22) This invention relates toradio-frequency loads, and more particularly, to a microwave water loadfor the dissipation of electromagnetic energy.

In the electronics industry it has become necessary to develop devicesto temporarily absorb the large amounts of power which are generated byelectron tubes and apparatus of various types, such as radar units, UHFtelevision transmitters, magnetrons, and klystrons. Because of the highpower capabilities of these units it is necessary to provide dummy loadswhich permit generation of the required energy and tuning of apparatuswhile precluding detectable radiation of the energy. Thus, in a militaryradar installation, particularly under battle conditions, it isnecessary to tune the radar equipment to an optimum power output. Theenergy generated must be dissipated or absorbed in some manner withoutradiating it into the atmosphere and thus disclosing the location of thetransmitter. For this purpose, Water loads have been found to be themost ideal dummy loads heretofore provided. It has been found that whenmicrowave energy is exposed to the pointed or tapered end of anattenuating medium, such as water, such energy is absorbed and convertedinto heat in the attenuating medium.

Because a rapid rise in the temperature of the attenuating body of wateroccurs, the container elements fluctuate in dimension due to thermalexpansion and contraction, thus creating problems of sealing the liquiddielectric or attenuating medium within the container. It is thereforean important object of the invention to provide a liquid attenuator orwater load designed to accommodate such thermal expansion andcontraction without destructive strains.

It has been determined that in the operation of a water load, thesharper the tip or cross-section of the tapered body of liquiddielectric or attenuating medium and the longer the taper, the lowerwill `be the VSWR, and the more eiiiciently will the load absorbradio-frequency energy. t is therefore still another object of theinvention to provide a water load, including a coaxial transmission lineportion, in which the taper is minimal at the electromagnetic energyinput end of the water load, and gradually increases in diameter to amaximum dimension equalling the interior diameter of the outer conductorof the coaxial transmission line.

The water load of the present invention is designed to absorb radiofrequencies ranging upward from about 200 megacycles. Water loadsdesigned to operate in this frequency range are normally required tohave a very long impedance transformer to provide the desired VSWR, andbecause of their length such transformers tend to be fragile. lt isaccordingly another object of the invention to provide a water loadwhich incorporates a long tapered container within a coaxialtransmission line, the tapered container being adapted to contain aliquid dielectric constituting an impedance transformer less fragilethan transformers of conventional waterloads.

To reduce undesirable reflections of radio-frequency energy admitted toa water load, it is desirable that the transformation ratio from thecoaxial transmission air line to the liquid dielectric coaxial sectionbe reduced to a minimum. Thus, in a water load for use with a SO-Ohrnline in which the inner conductor of the load has a constant diameter,the transformation ratio would be about 9 to 1. It is therefore anotherobject of the invention to provide the inner conductor of a water loadwith a configuration which will reduce the transformation ratio to about4 to 1.

It is a still further object of the invention to provide a water loadwhich may be constructed from standard coaxial transmission linematerials adapted to demountably receive a tapered dielectric container.

In using water loads to absorb radio-frequency energy, it is desirablethat the transition from the impedance of an air dielectric coaxialtransmission line to the impedance of the liquid dielectric coaxialsection of the line be as smooth as possible. It is therefore anotherimportant object of the invention to provide a coaxial water load inwhich the impedance is gradually reduced and the attenuatingcharacteristic of the load `section of the transmission line isgradually increased.

Inasmuch as the radio-frequency load must be asse i.- bled from rigidmechanical units having different dielectric constants, it is extremelydifficult to provide a smooth progressively increasing or decreasingcross section of liquid dielectric without discontinuities, bothelectrical and mechanical, in the linner and outer conductors whichadversely aifect the VSWR. It is therefore another object of theinvention to provide means for compensating the effect produced by theintroduction of materials having dilferent dielectric constants toprevent such differences fr'om adversely aifecting the dissipation ofenergy in the load.

1n a coaxial water load constructed according to this invention andcapable of dissipating over 50 kw. of radiofrequency power atfrequencies ranging between 225 and 1200 megacycles, the length mayreach approximately thirteen feet. ln water loads of this length it isdiiiicult to support the inner conductor throughout its length incoaxial relationship with the outer conductor. It is therefore anotherobject of the present invention to provide means for supporting theinner conductor at appropriate intervals, and compensating for thedifferent dielectric constant introduced by the support structure.

A still further object of the invention is the provision of a coaxialwater load which may be constructed in a short embodiment to absorbradio-frequency power at a high frequency limit, and which may beextended by the addition of easily attached extensions to make the loadapplicable to the dissipation of radio-frequency power at lowerfrequency limits.

The invention possesses other objects and features of advantage, some ofwhich, with the foregoing, will be apparent from the accompanyingdescription and drawings. lt is to be understood, however, that theinvention is not limited to the embodiment described and illustrated, asother forms may be used within the scope of the appended claims.

Brieily described, the radio-frequency water load of this inventioncomprises a length of coaxial transmission line having inner and outerconductors. At one end the outer conductor is provided with a flange fordetachably connecting the water load to a coaxial transmission line,while the associated end of the inner conductor is provided with aresilient coupling or connector for connecting the inner conductor ofthe water load to the inner conductor of the associated transmissionline. The opposite end of the water load is provided with an end capstructure having inlet and outlet ports for a liquid dielectric, such asWater or ethylene glycol.

The inlet port connects with the interior of the hollow tapered innerconductor, and liquid introduced into the inner conductor is dischargedtherefrom adjacent the R-F input end of the water load into a conicallytapered annular chamber surrounding the inner conductor and beingcontained and defined thereby and by a conical dielectric sleeve, rf`hesleeve at one end is connected to 4the inner conductor intermediate itsends in a manner to provide for the expansion and contraction of theparts due to increases and decreases in temperature. Means areassociated with this end of the sleeve to compensate for itsintroduction into the path of the electromagnetic wave. The taper of thesleeve and the taper of the inner conductor are proportioned to providean infinitely thin edge to the body of liquid at the R-F input end ofthe water load. The sleeve diverges toward the outer conductor of thewater load and terminates in a feather edge which provides a smoothtransition between the conical and cylindrical sections of the waterload. Means are provided for clamping and sealing the large featheredend of the dielectric sleeve in liquid-tight relation with the outerconductor.

Referring to the drawings:

FIGURE 1 is a horizontal half-sectional view showing the interiorconstruction of an extended version of the water load. A portion of thewater load is broken away to decrease its length.

FIGURE 2 is a fragmentary view of the end cap arrangement in a shortversion of the water load.

FIGURE 3 is a transverse sectional view taken in the plane indicated bythe line 3 3 in FIGURE l.

FGURE 4 is an enlarged sectional view showing an alternative method ofjoining two sections of the ex-` tended version of the water load.

FIGURES 1, 2, and 3 are drawn to a scale approximately one-half actualsize, and FIGURE 4 is drawn to a scale approximately full size.

Electromagnetic wave energy is dissipated in a dummy load by causing theelectromagnetic energy to encounter an attenuating substance, such aswater, which completely attenuates the wave by transforming theelectromagnetic energy into heat. To prevent undesirable refiections ofthe wave it is desirable that impedance change be effected gradually,with minimum attenuation occurring adjacent the R-F input of the loadand gradually increasing until maximum attenuation is effected.Attenuation at maximum value is continued until the transformation ofwave energy to heat is complete, which ideally occurs in advance of theterminal end of the load.

Coaxial water loads of the type illustrated are utilized to terminatecoaxial transmission lines along which electromagnetic energy ispropagated through the annular space between inner and outer conductors.To effectively attenuate the waves so propagated, a water load must,therefore, provide a body of an attenuating medium or liquid, preferablywater, in the annular space between inner and outer conductors, arrangedso that attenuation of the wave is effected gradually.

Minimum attenuation is effected where the cross-sectional area of theliquid dielectric first encountering the electromagnetic wave isminimal, and it is therefore desirable that the edge of the liquid whichthe electromagnetic Wave first encounters be as sharp as possible. Sincethe inner conductor of the water load extends for the full length of theouter conductor, the liquid must surround the inner conductor in 4theannular space between the inner and outer conductors and intermediatethe ends thereof. This necessitates that the liquid be contained in amanner to provide the requisite gradually increasing cross-section bymeans which are also transparent to electromagnetic energy.

Through extensive experiments it has been established that a verysatisfactory cross-sectional configuration for the body of liquiddielectric is one in which the sharpest j and leading edge of the bodyof liquid surrounds the inner conductor adjacent the R-F input end ofthe water load and gradually tapers away therefrom in oppositetransverse directions to a maximum cross-sectional area annular in formdefined by the inner diameterof the cylindrical outer conductor and theminimum outer diameter of the tapered inner conductor. The diameter ofthe thin leading edge of the liquid preferably approximates the meandiameter of the annular maximum cross-sectional area of the liquid.Electromagnetic energy entering the load thus passes through theelectromagnetic transparent means confining the liquid and is graduallytransformed into heat whichis dissipated by causing the liquidattenuator to circulate through the load.

The dim-cuit" of arranging rigid mechanical elements in a liquid-tightmanner to provide the body of liquid attenuator with the requisitetapered configuration, while at the same time preventing undesirablereflections of the electromagnetic energy will be readily appreciated.

To obviate these sealing and reflection problems, which are present inmost conventional water loads, the radiofrequency load of the inventioncomprises a cylindrical conductive container or metallic housing 2,having an R-F input end designated generally by numeral 3, and a liquiddielectric input-output end cap structure designated generally bynumeral 4. The R-F input end of outer conductor 2 is provided With aflange 6 for appropriate connection to a corresponding flange on acoaxial transmission line (not shown). At its other end the cylindricalconductive container 2 is provided with an end cap structure including abody portion '7 bored to provide inlet passage S adapted to be connectedby an inlet fitting 9 to a source of liquid dielectric. Egress of theliquid dielectric is permitted by outlet passage 12 in the body V7provided with an appropriate connector 13. A radial flange 14 on bodyportion 7 provides means for detachably securing the end cap structureto a complementary liange 15 on the end of the container -2 or anextension thereof as shown. A rubber O-ring 16 interposed between theflanges cooperates with bolts 17 to form a liquid-tight union of thefianges.

The inner conductor includes an input connector sleeve 1S detachablysupported intermediate its ends on the outer conductor 2 by an annulardieletric ring 19, and having at its forward end a plurality ofresilient fingers 29 for resilient mechanical and electricalinterconnection with the inner conductor of the transmission line towhich the water load is connected. At its other end the connector isprovided with similar resilient fingers which, in the drawing, have beenbroken away for the sake of clarity. Adjacent their base the fingersform cylindrically arranged tianges 21 providing a seat for one end of atubular inner conductor extension 22 which slips over the resilientngers in a snug sliding fit for good electrical continuity. The otherend of the tubular inner conductor extension is brazed to the rabbetedperiphery of contact block 23, preferably silverplated copper, whichalso forms a part of the inner conductor.

The contact block isdetachably connected by cap screw 26 to the closedor solid base end 27 of hollow conical inner conductor section 28.Contact block 23 is centrally bored to receive dielectric insulatorbushing 29 which serves to electrically insulate cap screw 26 from thecontact block. The end of the contactblock adjacent base end 27 of thehollow inner conductor section 28 is counterbored to provide a recessinto which a centrally disposed boss 31 on the end of base 27 is adaptedto extend. As shown in the drawing, the peripheral circular surface ofthe boss 31 is spaced from the complementary inner periphery of therecess in the contact block an amount to snugly receive a dielectricinsulator 32, which electrically insulates boss 31 from the contactblock.

The adjacent outer peripheral edges of contact block 23 and base 27 ofthe inner conductor 28 are provided with rabbeted recesses thereabout,the rabbet in base 27 being adapted to accommodate a deformable O-ring33, and the rabbet in the contact block being adapted to accommodate theinner radially fianged apex end 34 of the truncated hollow dielectriccone 36, preferably of Teflon, which constitutes a radio-frequencywindow Vand a liquid-impervious wall interposed between inner and outerconductors. As shown in FIGURE 1, cap screw 26 interposed betweencontact block 23 and base 27, draws these two parts together andeffectively binds the inner end of the dielectric cone Vbetween thedeformable O-ring 33 and the contact block, thussealing the .unionbetween the cone and the base 27 in a liquid-tight manner. Thecounterbored central recess in the contact block and the rabbetedperiphery thereof dene 'betweenthem acyli-ndrically extending ange l37which makes electrical contac with the base end 27 of the innerconductor when cap screw 26 is tightened. When the parts are assembled,it is this ange which iirst makes electrical contact with base 27 and,by appropriate means, such .as an ohm meter connected von opposite sidesof the joint, indicateswhen the flange 37 `has bottomed onto the end ofthe inner conductor base. The input connectorl is. detachably connectedto this assembly by Vmeans of capscrew 38 threaded into bore 39 inheadftt) ofv cap screw 26.

As previously discussed,` abrupt variations` in the crosssectionaldiameter of the spacevbetween inner conductor and outer conductor areapt to create undesirable dis- -turbances in the wave being transmittedtherethrough.

Thus, at the union of the contact block 23 -and the base end 27 wherethe two are' peripherally rabbeted, the Wave sees an abruptly largerspacing between inner and outer conductors because of the -groove formedinthe inner conductor at this point -by the juxtaposed rabbets. Tocompensate for this abrupt enlargement -of spacing, a di electric sleeveorrshell 4l having a tapered body is slipped over the inner conductorand is provided at `its end adjacent the groove with a radiallyextending liange .42 proportioned to reintroduce into the annular spacean appropriate amount of reactance to compensate the abrupt change indiameter of the inner conductor. The tapered body of the compensatorshell extends forwardly over the contact block Vand `terminates at itsend remote from flange 42 in a feathered edge ylying intermediate theends of extension 22,--thus eliminating the abrupt step at the endS14-of cone'36. A plurality of dielectric pins 43 driven through shell4i and into contact block 23 secure the compensator shell in position.

From its apex end 34, the dielectric cone 36 diverges outwardly to abase end V44 having a radially extending iiange 46. As shown inFlGURESl, 2, and 4, the outer conductor 2 is provided with a radially extendingintegral tlange 47 having a recess 43 formed around the inner peripherylthereof -within which radially'extending flange 46 may seat. Inembodiments of the water load which continue in length beyond thetransformer section defined by dielectric cone 36, an outer conductorextension 49 is provided with a radially extending iiange 51corresponding to iiange 47. Bolts 52, circumferentially spaced about andextending through the anges, bind the anges together.

To ensure water-tightness at tlt's union, a gasket 53 is providedbetween the anges and abutting the base end ange 46 of the dielectriccone. Since the inner diameter of the cone at its base 44 is somewhatsmaller than the inner diameter of the outer conductor at this point,this abrupt differential in diameters forms a step eliminated lby atapered dielectric shell 54, having a cylindrical outer periphery and aconical inner periphery forming a continuation of the conical innerperipheral surface of cone 36. A groove 56 in the outer periphery ofshell 54 cooperates with a plurality of dimples 57 formed in the outerconductor extension 49 to secure shell S4 in position. It will thus beseen that terminal end 58 of compensator shell 54, being feathered ortapered to the same diameter as the inner diameter of extension 49,provides a smooth transition for the electromagnetic wave from theconical section to the cylindrical section. Alternatively, compensatorshell 54 may be secured in outer conductor extension 49 by the construc-Y when empty.

tion shown in FIGURE 4, in which the compensator shell is provided vwitha radially extending flange 59 caught between ilanges 47 and 51.Water-tightness ofthe union is ensured by gasket 63 interposed betweenanges 46 and 59.

Brazed to the apex :end of the conical metallic inner conductor section2S `and forming an extension thereof, is .a tubular extension 66, ltheremote end 67 of which isslidably supported in 4a'liquid-tight mannerinrcentrally bored boss 68 extending from body 7 of end cap structure d.Suitable gasket means, such'as a deformable O-ring, interposed betweenthe bossand end 67 of tubular extension 66, provides -a water-tightunion while permittingrelative axial movement of the partsdue to thermalexpansion and contraction.

it will thus be seen that a liquid dielectric admitted through passage`8- passes successively through inner conductor'extension k66Vand'conical inner conductor section 28, andis discharged into theconically-tapered liquid container or chamber `.70 through passages orapertures 71 in base end 427 of theinner conductor. The coolest waterisA therefore admitted :to Vchamber 7,0 at its thinnest cross sectionand circulates backward `toward `the outlet passage 12.

4It has Vbeen .found through .experiment that a water load capable ofdissipating over 50 kw. ,of R-F power in the range between about 500 and1200 megacycles is only about 351/2 inches long between its mountingflanges, and 3% inches-in diameter, andweighs only 131/2 pounds Largerdiameters and lengths may, of course, be used .in ,accordance withfrequencyl ,and -power .dissipation-requirements. The uid ,capacity .ofa water load having the proportionsvnoted is approximately one- ,halfgallon andthe tlow to dissipate ,the power `indicated is .preferably ten.gallons per minute. 1t hasbeen .found that up to .about p.s.i. pressure,on the liquid dielectric canbe sustained bythe .water Vload of thisinvention, but to provide the requisite ow andA fiuid `temperature oftheliquid dielectric `the pressure should be. correlated` to the amount ofpower it is desired to dissipate. For example, to dissipateapproximately .50l kw., .with..a 20 C. rise in the temperature of theliquid dielectric, a riow rate of 10 g.p.m. .would require approximatelya tive-pound head. Tap -water may be used .as .the liquid dielectric, orif desired, ,deionized water, `Oria lsolution of V60% ethylene glycoland 40% deionized water, may be used.

In embodiments -of Ithe .water .load having a length up to about.13feet, itis desirable ithat` the inner conductor be supported atintervals Valong its length. Conventional supports for coaxialtransmission line, ihowever, are inadequate for inclusion in a water.load of thistype because they obstruct the tlow of liquid dielectricand also because they introduce at intervals material having a dierentdielectric constant than has the liquid dielectric and therefore causean abrupt increase in the impedance which introduces a sharp electricaldiscontinuity in the water load. To obviate these problems and stillprovide adequate support for the inner conductor, the inner conductor issupported by a dielectric spacer (FIGURE 3) having a hub 72 integralwith a plurality, preferably three, radially extending arms 73contacting the inner surface of the outer conductor. The hub of thedielectric spacer surrounds a metallic compensating ring 74 secured asby brazing to the inner conductor, the Spacer or hub being secured tothe compensating ring by a dielectric set screw 76. The metalliccompensating ring compensates for any abrupt discontinuity in impedancecaused by introduction of the dielectric spacer. To lighten the spacerand also to remove as much material as possible to lessen the effect ofthe spacer as a deterrent to the ow of liquid through the water load,and also to minimize the low dielectric medium to lessen the electricaldiscontinuity caused thereby, the arms 73 are preferably apertured asshown.

From the foregoing it will be seen that with water ad- 7 Y mitted to thewater load and passing therethrough at approximately 10 gallons perminute, and with radio-frequency energy admitted at the input end, anincrease in the temperature of the water will be reflected by anincrease in temperature of the mechanical elements which channel andcontain it. Such increase or decrease in temperature of the mechanicalelements will result in expansion and contraction of these parts.Inasmuch as the outer conductor 2 will remain at substantially constantor ambient temperature, or perhaps cooler, it is important that elementsof the combination which expand and contract with increase or decreasein temperature be arranged in a manner to prevent destructive stresses.The inner conductor being metallic, and the base end of the dielectriccone 36 being anchored in position, it will be obvious that these twoelements will expand and contract at different rates. It is thereforeimportant that the input end of the inner conductor be slidablysupported in the insulator 19. It is likewise important that theterminal end 67 of the inner conductor in either the short or extendedembodiments be slidably supported in the boss 68. Fluctuations intemperature, and consequent expansion and contraction will then resultin the apex end of the dielectric cone being caused to oat so that itassumes a position determined by its own expansion and contraction rate,but the metallic elements extending in opposite directions from the apexend ofthe cone will be slidably displaced axially relative to theirrespective supports. From this it is seen that no appreciable stresswill be placed on the dielectric cone 36resulting in a long lifetherefor and liquid-tight joints for the length of such life.

In tuning equipment to which the water load is connected, it is oftendesirable to sample the power being admitted to the water load. For thispurpose a coaxial outer conductor tting 77 is fixed to the outerconductor 2 adjacent its input end and in advance of the cone 36. Thefitting permits the insertion of a convenient sampling device, such as acoupling loop (not shown), to sample the power being admitted to thewater load.

I claim:

1. In a radio-frequency load for coaxial transmission lines, thecombination comprising a hollow outer housing, an inner conductorlocated within the outer housing and slidably supported adjacent each ofits ends spaced from the outer housing, dielectric wall means sealinglyinter,`

posed between the inner conductor and outer housing intermediate theends thereof and therewith dening a chamber for a liquid dielectric, andtapered shell means adjacent opposite ends of said well means formingextensions thereof merging smootlhy and respectively in contact with theassociated inner conductor and outer housing.

2. A radio-frequency load for radi0-frequency transmission linescomprising a hollow outer housing, an inner conductor extending throughthe outer housing and including a hollow conical portion closed at oneend and having a plurality of apertures adjacent the closed end thereof,a contact block detachably secured to the closed end of the hollowconical inner conductor portion and constituting an extension thereof, aconnector detachably supported on the housing and conductively connectedto thecontact block, dielectric wall means comprising a truncatedconical sleeve sealingly interposed between the inner conductor andouter housing and therewith defining a chamber communicating with theinterior of said inner conductor to admit a liquid dielectric into thechamber, an end cap structure closing one end of the chamber and havingan outlet port communicating therewith, and tapered dielectric shellmeans adjacent opposite ends of said wall means forming extensionsthereof merging smoothly with and respectively in contactV with theassociated inner conductor and outer housing.

3. The combination according to claim 2, in which one of said tapereddielectric shells is secured to the inner conductor adjacent the apexend thereof and another of said shells is secured to the outer housingadjacent the other end of the conical sleeve, and means for supportingsaid inner conductor at spaced intervals from said outer housing, saidsupporting means comprising a dielectric spacer having a hub encirclingsaid inner conductor, said spacer having a plurality of radiallyextending arms integral with said hub, said arms being in contact withthe inner surface of said outer housing and provided with apertures forpermitting passage of said liquid dielectric therethrough.

References Cited by the Examiner UNITED STATES PATENTS 2,848,683 8/58Jones S33-22 2,881,399 4/59 Leyton 333-22 2,894,219 7/59 Federico 333-222,946,005 7/ 60 Watereld et al. 333-22 3,044,027 7/62 Chin et al 333-22OTHER REFERENCES Freedman: Water Loads, Radio-Electronic Engineering,

0 pages 14, 15, and 35, May 1954.

HERMAN KARL SAALBACH, Primary Examiner.

ELI I. SAX, BENNETT G. MILLER, Examiners.

w fun-B.,

1. IN A RADIO-FREQUENCY LOAD FOR COAXIAL TRANSMISSION LINES, THECOMBINATION COMPRISING A HOLLOW OUTER HOUSING, AN INNER CONDUCTORLOCATED WITHIN THE OUTER HOUSING AND SLIDABLY SUPPORTED ADJACENT EACH OFITS ENDS SPACED FROM THE OUTER HOUSING, DIELECTRIC WALL MEANS SEALINGLYINTERPOSED BETWEEN THE INNER CONDUCTOR AND OUTER HOUSING INTERMEDIATETHE ENDS THEREOF AND THEREWITH DEFINING A CHAMBER FOR A LIQUIDDIELECTRIC, AND TAPERED SHELL MEANS ADJACENT OPPOSITE ENDS OF SAID WELLMEANS FORMING EXTENSIONS THEREOF MERGING SMOOTHLY AND RESPECTIVELY INCONTACT WITH THE ASSOCIATED INNER CONDUCTOR AND OUTER HOUSING.